Sprag clutches with a plurality of sprags positioned between an inner race and an outer race are well known. Typically, the sprags are retained within a cage that rotates together with one of the races. Sprag clutches which use a centrifugal liftoff action to effect liftoff of the sprags from either the inner race or the outer race are also known. The sprags are typically biased by a spring into engagement with both races to effect a wedging action, or lockup, to prevent rotation of one of the races relative to the other in a first direction. Relative rotation of the races with respect to each other in a second direction, called overrunning, is permitted. At a certain angular velocity of a race in the second direction, the centrifugal force acting on the sprags due to an asymmetric geometry of each sprag reaches a level that causes the sprags to pivot about an axis of rotation such that a clearance develops between the sprags and the races. The clearance, or rollover, removes the torque coupling between the races.
Smooth rotation and concentricity of the races are controlled by a plurality of bearings placed adjacent to the sprags between the races. Bearing inner diameter corresponds with and defines a portion of an outer diameter of the inner race. Similarly, bearing outer diameter corresponds with and defines a portion of an outer race inner diameter. Precision bearings are available in many sizes and in many increments of outer diameter. Therefore, many bearing diameters are possible in a family of clutches.
Bearing radial thickness is usually larger than a corresponding radial thickness of a sprag placed between the races. Known clutch designs account for the difference between bearing radial thickness and sprag radial thickness by machining steps in both inner race outer diameter and outer race inner diameter. When assembled, the steps cooperate to define a radial gap corresponding to sprag radial thickness that is smaller than a radial gap formed to accommodate a bearing radial thickness. Thus, in conventional clutches, both races include a bearing diameter as well as a different sprag diameter. Such a configuration is expensive and time consuming to machine and manufacture, requiring tight tolerances.
In some sprag clutches, the radial gap between the races defined by the steps may be filled with sprags in positive continuous engagement. Sprag positive continuous engagement means that each sprag is in contact with adjacent sprags before rollover of the sprag into a lockup position, and each sprag maintains that contact in the lockup position. Positive continuous engagement is enabled by designing a sprag geometry such that one radially extending side of the sprag includes a nose portion which contacts a second radially extending side of a second, adjacent sprag. Sprag circumferential width between the contact points of a single sprag defines a sprag arc-length, while the contact points of all sprags define a sprag contact diameter along which adjacent sprags are positively continuously engaged.
While conventional sprag clutches may make use of a single sprag design and size to improve design efficiency, each clutch must be designed with particular and distinct bearing diameters and sprag diameters. In particular, race diameters, sprag diameters, and bearing inner and outer diameters all impact design of a clutch and step height. Therefore, changing race diameters requires customization of bearing diameters, sprag diameters or both to utilize identical sprags in each clutch, creating design and manufacture difficulties.